A new type of flying optical head combines elements from a broad range of technologies including fiber optics, micromachining, Winchester-style slider fabrication, and micro-optics. Besides the initial technological challenge of creating laboratory prototypes of the optical head, there is the additional formidable task of transferring the technology to manufacturing. For production of the flying optical heads to be successful and profitable, the process for their manufacture needs to be inexpensive, repeatable, and well characterized. Testing of the various components of the optical head at strategic points in the assembly process is desired for the characterization of the manufacturing process.
A key component of the optical head introduced above is a steerable micro-machined mirror that has been fully described in commonly assigned patent application "Data Storage System Having an Improved Surface Micro-Machined Mirror" (U.S. Ser. No. 08/844,207). The micromachined mirror should be tested at the component level prior to assembly onto the optical head. Among the battery of tests that should be performed are the following characterizations:
Functionality: Verification that the steerable micro-machined mirror moves when an actuation voltage is applied to drive electrodes.
Voltage Sensitivity: Characterization of a sensitivity of a deflection angle of the steerable micro-machined mirror as a function of an applied actuation voltage.
Mirror Dynamics: Measurement of modal frequency for torsional excitation of the mirror and its quality factor, Q, for this mode.
Mirrors that pass the desired performance specifications may be used for assembly onto the flying optical head. However, during assembly of the head, the mirror may become damaged and consequently will no longer function as required. Thus, the mirror would need to be retested during and/or after assembly onto the head. In order for assembly costs to be minimized, it is important to identify damaged mirrors as early in the assembly process as possible such that labor is not wasted on non-functioning heads. The small form factor of the head, especially when it is assembled onto a head stack, makes retesting of the steerable micro-machined mirror especially challenging.
Torsional aluminum actuators have been tested on a benchtop environment for angular deflection and dynamics using standard optics as described in a paper by K. Honer, et. al., "A High-Resolution Laser-Based Deflection Measurement System for Characterizing Aluminum Electrostatic Actuators" (Proceedings of Transducers 1995; Stockholm, Sweden; pp. 308-311). This paper describes laser light that is produced by a He--Ne laser with a 500 .mu.m beam diameter and directed through an attenuator, a beam splitter, and a lens that focuses a spot onto a surface of the actuator. Light reflected off of the actuator surface passes back through the lens and is diverted by the beam splitter onto two photodiodes positioned 100 .mu.m apart. Upon actuation, the reflected spot position with respect to the position of the two photodiodes is detected as a difference in illumination between the two photodiodes. Reflected spot movement with respect to its position for the undeflected actuator is a function of actuator angular rotation and the focal length of the lens. Standard optical and electronic methods are used to deduce the voltage sensitivity and dynamics of the actuator from measurements of the movement of the reflected spot.
Testing of dynamic optical elements, such as steerable micro-machined mirrors, at the optical head stack level is not addressed by Honer et al. Additionally, Honer et al. rely on normal incidence of a laser light onto an actuator, which is not available on a flying optical head if a steerable micro-machined mirror is positioned at a 45 degree angle with respect to the incoming light's incident propagation axis. Much of the test method of Honer et al. relies on the design and construction of an externally based optical path. The Honer et al. approach does not take into consideration the requirements imposed by the optical path, associated steerable micro-machined mirror, and head stack assembly as described in concurrently assigned patent application Ser. No. 08/844,207. The use of a photodiode pair by Honer et al., makes this test method inherently one-dimensional, that is, only spot movement along the axis defined by the two photodiodes can be detected; transverse movement of the spot cannot be detected.
What is needed, therefore, is a method for testing the performance of dynamic components including micro-machined mirrors that are mounted to an optical head. This method must be compatible with the need for testing the components at various points in the optical head assembly process including, most critically, when an array of such heads has been assembled onto a head stack assembly. The method should take into account and deal with the limited clearance on the head stack available for any components required to perform the test.